Hydraulic braking systems are equipped in the majority of all passenger vehicles in the market. In a hydraulic braking system, a brake pedal is mechanically coupled to master cylinder that delivers pressurized fluid to wheel brakes as the brake pedal is depressed. A vacuum booster can also assist the movement of the brake pedal to reduce the brake pedal application force necessary to deliver hydraulic fluid to the brakes.
A brake-by-wire system is a brake actuation system in which the brake pressure generation is mechanically decoupled from the brake pedal. A brake-by-wire system may use a sensor to determine position of the brake pedal, pedal application pressure, or both. The brake pedal position is processed by an appropriate controller to generate a corresponding braking request. The braking request is used to control an associated actuator which applies hydraulic/pneumatic or electric signals to wheel brake devices. As there is no direct mechanical connection between the brake pedal and the wheel brake devices, the braking controller may determine an appropriate amount of braking torque to be applied to the wheels of the vehicle based upon the position of the brake pedal.
Vehicles equipped with brake-by-wire systems can also have a hydraulic braking system as a backup to the by-wire system. In direct response to an inability to detect the driver's braking demands, a “push-through” mechanical hydraulic braking mode can be activated in which hydraulic pressures are applied to the braking devices by means similar to conventional hydraulic braking systems.